The present application relates to area array packages, and more particularly to increasing the reliability of solder joints of an area array package subjected to environmental stresses.
Semiconductor integrated circuit chips have to be connected in order to enable them to interact electrically with the outside world. A ball grid array (BGA) semiconductor chip package employs a plurality of solder balls as external terminals. A BGA package is widely employed because it allows a multi-pin structure over a limited area.
Chip devices typically have a coefficient of thermal expansion (CTE) of about 3 ppm/C. These devices are relatively stiff and can fracture in a brittle manner if stressed by excessive bending. An area array package includes a chip, a package substrate, and optional molding. The CTE of the area array package is affected by each component of the package.
An epoxy-glass printed circuit board can have a CTE in the range of about 16 to 21 ppm/C, depending on the glass cloth, resin system, and copper content. It is necessary to provide a physical connection between the area array package and the printed circuit board in order to obtain a useful electrical connection.
A mismatch in CTE that exists between an area array package and a printed circuit board contributes to thermally driven stress and can affect package reliability in many ways. In some manner, all electronics packaging schemes involving an area array package and a printed circuit board are affected by this fundamental mismatch in CTE. Measurements on semiconductor packages of the ball-grid array (BGA) type and chips-size (CSP) type found that solder joints located close to package corners are under particularly heavy strain due to mismatch in CTE.
As the land pad pitch and size shrinks, solder joint reliability is typically decreased. The area array package requires a packaging scheme to form and maintain electrical contacts between the package and a printed circuit board over the entire face of a device. Temperature-dependent shear strains exist between the area array package and the printed circuit board. There is a very predictable, finite fatigue life of solder ball connections. Additionally, area array packages are also subject to mechanical stresses not related to the CTE. These stresses include mechanical bending and other environmental stresses.
Stresses between the area array package and the printed circuit board leads to the need for protection of solder balls that provide electrical connection. The protection of the solder balls that provide electrical connection should provide sufficient relief such that strains on solder connections are reduced to acceptable levels so that fatigue life improvement is realized.
An area array package includes a plurality of solder balls not used as electrical connectors. These non-connected solder balls, or xe2x80x9cdummy balls,xe2x80x9d provide protection to solder balls connected to live pins and therefore increase reliability of the area array package. The dummy balls may be placed in the corners, along the diagonals or in other high stress locations on the area array package. To further increase reliability, a continuous copper ball land pad may be used to connect each group of corner dummy balls. Continuous copper pads help to reduce stress on the dummy balls. For center-depopulated BGA packages, an array of dummy balls may be used in the center of the package to prevent substrate bending and improve drop test reliability.
Improving the reliability of area array packages using dummy balls and advantages of the embodiments will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings.